1. Field of the Invention
The present invention relates to an oxygen concentration cell, which operates even at a lower temperature and has a more rapid response speed, and a method of producing the same.
2. Description of the Prior Art
It has been known that a sintered article of zirconium oxide stabilized with CaO, Y.sub.2 O.sub.3 or the like is an oxygen ion conductive solid electrolyte having an oxygen ion transport number of about 1.0, and that an element consisting of the oxygen ion conductive solid electrolyte provided on its surfaces with porous platinum electrodes or the like serves as an oxygen concentration cell, which is used as a sensor for measuring the oxygen partial pressure in the waste gas of a boiler or in the exhaust gas of an internal combustion engine, or is used as a fuel cell.
When an oxygen concentration cell is used as an oxygen sensor, the cell is required to generate an electromotive force at a lower temperature and to have a more rapid response speed relative to the variation of oxygen partial pressure. Particularly, when the cell is used as a sensor for measuring the oxygen partial pressure in the exhaust gas of an internal combustion engine, the cell is required to have electrodes tightly adhered to the cell, because the cell is exposed to thermal shock and high speed exhaust gas.
A simplified electric equivalent circuit of an oxygen concentration cell can be expressed by a series circuit consisting of the interfacial impedance between electrodes and solid electrolyte, the impedance of solid electrolyte itself and the electromotive force. The impedance of solid electrolyte itself depends substantially upon its composition when the electrolyte is sufficiently densely sintered, and the electromotive force is determined by the well-known Nernst equation. However, it is known that the interfacial impedance is highly influenced by the kind of materials for the porous electrode and the applying method of the electrode. Therefore, in order to obtain an oxygen concentration cell capable of generating an electromotive force at a lower temperature and having a more rapid response speed, it is necessary to provide porous electrodes having a proper amount of pores and capable of giving a low interfacial impedance.
Platinum group metals are generally used as the electrode of oxygen concentration cells of this kind, and as the method of applying an electrode formed of platinum group metal, there have been known chemical plating methods, physical plating methods, baking methods and the like. As the oxygen concentration cell is provided with electrodes capable of generating an electromotive force at a lower temperature, that is, electrodes having a low interfacial impedance, there has been known an oxygen concentration cell provided with cermet-like electrodes consisting of a sintered mixture obtained by applying a mixture consisting of powders or finely divided particles of a platinum group metal and powders or finely divided particles of an oxygen ion conductive solid electrolyte to an oxygen ion conductive solid electrolyte substrate, and baking the mixture to the solid electrolyte substrate. However, the cermet-like electrode of this oxygen concentration cell is generally baked at a temperature of about 1,300-1,400.degree. C. in order to generate an electromotive force at a lower temperature and to obtain a higher response speed. However, when a baking is effected at such temperature, the solid electrolyte powder in the mixture is not integrally sintered with the solid electrolyte substrate, and the resulting electrode is poor in the mechanical strength and is easily exfoliated by mechanical friction. While, when a baking is carried out at a temperature of not lower than 1,4000.degree. C. in order to obtain an electrode having a higher resistance against mechanical friction and a higher adhesion strength, the resulting cell generates an electromotive force at a lower temperature, but is low in the response speed. Accordingly, the development of an oxygen concentration cell capable of generating an electromotive force at a lower temperature and having a higher response speed has been eagerly demanded.